they turn out to have not two, but at least five eyes

It can be explained that the discovery of these extra eyes took so long. They are – in the fossilized trilobites – somewhat hidden.

About five million years ago, the oceans were teeming with trilobites. This is a well-known class of extinct arthropods that lived in the sea in ancient times. Trilobites, along with ammonites and dinosaurs, are undoubtedly among the best-known fossils. But despite 150 years of research, the creature still has surprises in store for us. For example, scientists encountered additional eyes that were overlooked from the start.

Eyes
Arthropods generally have two types of eyes. First, they have a pair of compound eyes (also called compound eyes). These often consist of thousands of separate lenses. Moreover, arthropods are also equipped with median eyes. It is actually an additional set of simple eyes, located in the middle of the forehead, between the compound eyes. Only the trilobites, a large group of arthropods that lived in the Paleozoic era, seemed oddly lacking median eyes.

Discovery
At least, that was the assumption until recently. Until researchers discovered the trilobite Aulacopleura koninckiipart of whose head had been scraped off, subjected them to closer inspection.

The team found three small, nearly identical, dark, inconspicuous oval spots of the same size on the front of the head. These three structures are parallel to each other and fan out slightly downward. All three spots are characterized by a smooth, clear outline and a uniform dark brown color. “These spots are clearly distinct from the spots created by decomposition or fossilization,” explains researcher Brigitte Schoenemann. “Instead, it’s exactly what simple median eye remnants look like.”

middle eye
This means that trilobites – like all other arthropods and many close relatives – could not only have compound eyes, but were also equipped with an extra pair of eyes. “The finding supports the hypothesis that trilobites originally had median eyes,” Schoenemann concludes.

Unnoticed
It is remarkable that the trilobite managed to keep this secret for so long. As mentioned, the trilobite has been widely studied for over a century. In addition, more than 22,000 species have already been discovered. Despite this, the middle eyes have gone unnoticed all this time. Nevertheless, it can be explained. They are – in the fossilized trilobites – somewhat hidden.

Constipated
Scientists assume that the median eyes were characteristic of the larval stage, they explain their study out. In addition, they were located under a transparent layer of the shield. During the process of fossilization, however, this layer becomes opaque. “These two elements contributed to the fact that the discovery of the extra pair of eyes was so long overdue,” Schoenemann said.

Learn more about trilobites
Trilobites can be recognized by a calcified exoskeleton on the back of the body. They had a crescent-shaped head that resembled today’s horseshoe crab. Although the animals are now extinct, they were an evolutionarily successful species. They have survived for over 250 million years; longer than the dinosaurs.

This means that the trilobites probably had several hidden eyes. When researchers discovered the trilobite Sibilla cyclopyge they also found three median eyes on the forehead, just between the two compound eyes. Moreover, these were even equipped with lenses similar to those of human eyes. This indicates that this trilobite probably had better eyesight than its closest relative. Aulacopleura koninckiiwho – unlike Sibilla cyclopyge – only lived downstairs.

Variant
In short, the results show that trilobites had many more eyes than previously thought; probably at least five! Although this number may also have varied. Researchers believe that arthropods had different numbers of median eyes at different times in evolution. “The initial median number of eyes is two,” says Schoenemann. “Also, very early arthropods probably had four. Contemporary animals, such as insects and crustaceans, are equipped with three median eyes.

In total, thanks to their own sharp eye, the researchers managed to solve the mystery of the missing median eyes in trilobites. But that doesn’t just broaden our knowledge of these extinct prehistoric marine animals. The fact that the number of median eyes varied among arthropods could potentially aid the evolutionary classification of archaic arthropods. “By using the number of median eyes, we can now more easily determine the position of an arthropod in the evolutionary tree,” concludes Schoenemann.